External transformer correction in an electricity meter

ABSTRACT

A method compensates for measurement errors of an external transformer coupled between an electricity meter and one or more power lines. The method includes a first step of obtaining at least one error rating for the external transformer. The method also includes the step of storing data representative of the at least one error rating in a memory within the meter. At some point the electricity meter is coupled to the external transformer. The method further includes the step of employing the meter to obtain at least one electricity consumption measurement value, the at least one electricity consumption value comprising either a sampled current value or a sampled voltage value. Finally, the method includes the step of causing the meter to adjust the at least one electricity consumption measurement value using at least a portion of the stored data.

This application claims is a continuation of U.S. patent applicationSer. No. 09/693,025, filed Oct. 20, 2000, which claims the benefit ofU.S. Provisional Patent Application No. 60/160,675, filed Oct. 21, 1999.

FIELD OF THE INVENTION

The present invention relates generally to electricity meters, and inparticular, to electricity meters that are coupled to a system to bemeasured via external transformers.

BACKGROUND OF THE INVENTION

Electricity meters are devices that, among other things, measure theconsumption of electricity by a residence, factory, commercialestablishment or other such facility. Electricity meters may also beemployed to measure electricity flow between different electrical powergeneration or delivery systems. Electrical utility service providersemploy electricity meters to track customer usage of electrical power orenergy. Utilities track customer usage for many purposes, includingbilling and tracking demand.

At present electricity meters often include digital circuitry thatperforms many of the energy-related calculations. Examples of suchmeters are shown and discussed in U.S. Pat. No. 5,544,089, U.S. Pat. No.5,933,004, and U.S. Pat. No. 6,112,158, which are incorporated herein byreference. Such meters, referred to generally herein as electronicmeters, typically include analog sensor circuitry and digitalmeasurement circuitry.

The analog sensor circuitry includes one or more sensor devices thatsense or detect current and/or voltage levels on the power lines of thesystem being measured. The sensor devices generate analog measurementsignals that are representative of the detected current and voltagewaveforms actually present on the power lines. The analog sensorcircuitry typically provides the signals to analog to digital conversioncircuitry which generates digital versions of the measurement signals.

In an electronic meter, the measurement circuit typically performsenergy-related calculations on the digitized measurement signals. Ingeneral, the measurement circuit will perform, among other things, anenergy or watt-hour calculation.

To this end, measurement circuit multiplies the digitized voltagemeasurement signal by the digitized current measurement signal. Inparticular, the digitized measurement signals consist of sampled voltagemeasurement values and sampled current measurement values. Bymultiplying the individual voltage samples by the individual currentsamples and summing the resulting products over time, energy consumptionvalues are obtained. In particular, in a single phase system, the energyconsumption measurement may be given by the following equation:WH=ΣV(n)*I(n)*T _(n); for n=1 to N,Where WH is equal to energy consumption (e.g. watt-hours), T_(n) is thesample period for n, V(n) is the nth voltage sample, and I(n) is the nthcurrent sample that is sampled contemporaneously with V(n).

The measurement circuit thereafter typically displays or communicatesthe calculated watt-hour data to a centralized computer or the like. Themeasurement circuit may also perform intermediate calculations toconvert the WH data into units of measurement typically used in themetering industry. The measurement circuit may further perform variousdata tracking operations and/or control relays or other external devicesresponsive to the measured data. It is noted that the measurementcircuit may calculate other energy consumption measurement values, suchas RMS current, RMS voltage, reactive energy, apparent energy, orvarious power values. Such measurement values, as well as other energyor power related values generated from the measurement signals, arereferred to generically herein as energy consumption data.

One important element of electricity meters, including electronicmeters, is metering accuracy. Metering accuracy is important becauseinaccurate metering can result in substantial amounts of lost revenue.Moreover, inaccurate metering can also undesirably result inovercharging of customers.

The common sources of metering inaccuracies, or error sources in ameter, include the sensor devices in the sensor circuitry. Inparticular, sensor devices can produce error in both the magnitude ofthe measurement signals and the phase of the measurement signalwaveform. For example, current transformers, which generate currentmeasurement signals, often introduce significant magnitude and phaseerror into the current measurement signal. Such errors propagate throughto the calculated energy consumption data.

To reduce the errors due to the sensor devices, electronic meters aretypically calibrated. In particular, it is known to introduce acalibration factor into the energy calculation to compensate for currenttransformer error. The calibration factor is typically determined bysubjecting the meter to a calibration procedure in the factory. Thecalibration procedure involves attaching the meter to measure a knownamount of energy. The calibration factor is derived from the differencein the known amount of energy delivered and the amount of energyactually registered by the meter.

While such calibration procedures may reduce the inaccuracy due to errorsources within the meter, factory calibration procedures are inadequatefor addressing error sources external to the meter. One external errorsource is an instrument transformer. An instrument transformer is atransformer that is connected between the meter and the power lines.Instrument transformers are used to scale down the voltage and/orcurrent that is actually delivered to and measured by the meter.Instrument transformers are uncommon in single residence applications,but are relatively common in larger, higher voltage and current systems,such as those used for large industrial or commercial establishments.

Instrument transformers, like the current transformers within the meter,often exhibit notable phase error and magnitude or ratio error. Indeed,it is common in the instrument transformer industry to provide a visualindication of the phase error and ratio error data on the instrumenttransformer itself. Such data are referred to as error ratings,including phase error ratings and ratio error ratings.

Because instrument transformers are external to the meter, factorycalibration of the meter to compensate for the instrument transformerratio and phase error is impracticable. In particular, instrumenttransformers are often installed independently of the meter, and indeedmay be supplied from a different supplier than the electricity metersupplier. Thus, current calibration procedures cannot be used tocalibrate the meter for the error caused by the external instrumenttransformers.

Errors due to instrument transformers are not insignificant.Accordingly, inaccurate metering and billing due to ratio and/or phaseerrors in instrument transformers can cost utilities and customerssignificant amounts of money. As a result, there is a need for somemethod (and apparatus) that corrects or compensates for errors caused byinstrument transformers, or any transformer located external to themeter, to ensure relatively accurate metering.

SUMMARY OF THE INVENTION

The present invention fulfills the above need, as well as others, byproviding a meter that stores information representative of errorratings of an external transformer, and then adjusts energy consumptionvalues based at least in part on the stored information. Because theenergy consumption values are adjusted based on the stored informationrepresentative of the error ratings of an external transformer, themeter can adjust the its energy consumption calculations to compensatefor errors caused by the external transformer.

An exemplary method according to the present invention compensates formeasurement errors of an external transformer coupled between anelectricity meter and one or more power lines. The method includes afirst step of obtaining at least one error rating for the externaltransformer. The method also includes the step of storing datarepresentative of the at least one error rating in a memory within themeter. At some point the electricity meter is coupled to the externaltransformer. The method further includes the step of employing the meterto obtain at least one electricity consumption measurement value, the atleast one electricity consumption value comprising either a sampledcurrent value or a sampled voltage value. Finally, the method includesthe step of causing the meter to adjust the at least one electricityconsumption measurement value using at least a portion of the storeddata.

An exemplary apparatus embodiment according to the present invention isan apparatus for use in an electricity meter that is operably coupledthrough an external transformer to measure electricity consumption on apower line. The apparatus is operable to compensate for measurementerrors of an external transformer, and includes a memory and aprocessing circuit. The memory stores data representative of at leastone error rating for the external transformer. The processing circuit isoperable to obtain at least one electricity consumption measurementvalue, the at least one electricity consumption measurement valuecomprising either a sampled current value or a sampled voltage value.The processing circuit is further operable to adjust at least oneelectricity consumption measurement value using at least a portion ofthe stored data.

Accordingly, the above described method and apparatus compensate forexternal transformer error by adjusting energy consumption measurementvalues based at least one of the external transformer's error ratings.Moreover, as will be discussed further below, because the aboveembodiment of the present invention adjusts the actual voltage and/orcurrent sample values, the compensation will propagate through to allenergy consumption calculations that use the voltage and/or currentvalues. By contrast, systems that adjust the watt-hour calculationsalone may still use the erroneous raw data for other types ofcalculations within the meter.

Another exemplary method according to the present invention compensatesfor measurement errors of an external transformer coupled between anelectricity meter and a power line, the electricity meter operable tomeasure electricity consumption on the power line. The method includesthe steps of obtaining at least one error rating for the externaltransformer and storing data representative of the at least one errorrating in a memory within the meter. At some point the electricity meteris coupled to the external transformer. The method further includes thesteps of employing the meter to obtain at least one electricityconsumption measurement value, and causing the meter to multiply eitherthe at least one electricity consumption measurement value or a phaseshifted electricity consumption measurement value by a dynamiccompensation factor.

The above described method provides an advantage in that the amount ofthe adjustment is dynamic, and changes as the level of power consumedchanges. As a result, the meter can compensate for an error of thetransformer that varies as a function of the power (or current) throughthe transformer. This advantage can increase metering accuracy,regardless of whether the compensation is applied to the voltage and/orcurrent samples or the final energy consumption calculations.

It is moreover preferable to decrease the adjustment factor as the powerconsumption level increases. To this end, the adjustment may involve theuse of a correction factor that has an inverse relationship to anaverage level of current measured on the power lines. Such an adjustmentapproximates the error response curve of external instrumenttransformers.

The above described features and advantages, as well as others, willbecome more readily apparent to those of ordinary skill in the art byreference to the following detailed description and accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic block diagram of an exemplary electricity meterthat incorporates a compensation apparatus according to the presentinvention installed to measure a three phase power system;

FIG. 2 shows an exemplary diagram of a reduced power line voltagewaveform over time;

FIG. 3 shows a block diagram of operations performed on current samplevalues in an exemplary compensation apparatus according to the presentinvention;

FIG. 4 shows a block diagram of operations performed on voltage samplevalues in an exemplary compensation apparatus according to the presentinvention;

FIG. 5 shows error versus load current curves for an exemplarytransformer; and

FIG. 6 shows an exemplary phase shifter that may be used in connectionwith the flow diagrams of FIGS. 4 and 5.

DETAILED DESCRIPTION

FIG. 1 shows a schematic block diagram of an exemplary electricity meter10 that incorporates a compensation apparatus 12 according to thepresent invention. The electricity meter 10 is shown in contextinstalled in a three phase power system. The exemplary embodiment of thecompensation apparatus 12 described herein compensates for measurementerrors caused by three external current transformers (“CTs”) 14, 16, and18, and three external potential transformers (“PTs”) 15, 17 and 19. TheCTs 14, 16 and 18 and the PTs 15, 17 and 19, respectively cooperate tocouple the electricity meter 10 to the three phase power lines 20, 22,and 24, respectively.

While the electricity meter 10 in FIG. 1 is configured to measure athree phase, four wire wye service connection as is known in the art, itis be appreciated that those of ordinary skill in the art may readilymodify the arrangement of FIG. 1 to accommodate a three wire delta, fourwire delta, or other standard service connection types. The electricitymeter 10 and the compensation apparatus 12 may also be readily modifiedfor use in single phase service connections.

The meter 10 includes a source of digital energy consumption signals 26,a processing circuit 28, a memory 30, a display 32 and an input device34. The meter 10 may of course include additional elements as a matterof design choice. For example, the meter 10 may include one or moreexternal communication circuits. Likewise, the meter need not include adisplay 32 or an input device 34, depending on the embodiment employed.

In general, the compensation apparatus 12 includes at least a portion ofthe functionality of the processing circuit 28 and the memory 30.However, as is known in the art, the processing circuit 28 and thememory 30 can be configured to carry out several functions within theelectricity meter 10, in addition to those attributable to thecompensation apparatus 12 as described herein. Indeed, in the exemplaryembodiment described herein, the processing circuit 28 furtherconstitutes the main measurement circuit of the meter 10 in addition toits operations as part of the compensation apparatus 12.

In general, the power lines 20, 22, and 24 and a neutral line 38 areoperably coupled to provide three-phase power to from a utility serviceprovider, not shown, to a load 36. The CTs 14, 16 and 18 are currentinstrument transformers that deliver reduced current signals from powerlines to an electricity meter. Such devices are well known in the art.Likewise, the PTs 15, 17 and 19 are potential instrument transformersthat deliver reduced voltage signals from the power lines to anelectricity meter. Such devices are also well known.

As shown in FIG. 1, the first external CT 14 and the first external PT15 are coupled to the phase a power line 20. The first external CT 14 isconfigured to provide a reduced phase A power line current signal,RILS_(A), which is representative of the current waveform on the phase Apower line, to the source of digital energy consumption signals 26. Thefirst external PT 15 is configured to provide a reduced phase A powerline voltage signal, RVLS_(A), which is representative of the voltagewaveform on the phase A power line, to the source of digital energyconsumption signals 26. In general, the first external CT 14 and thefirst external PT 15 introduce phase error and ratio error into thereduced phase A power line signals during the conversion. As is known inthe art, the error ratings for the CT 14 and the PT 15 may be providedthrough indicia printed thereon.

Similarly, the second external CT 16 is configured to provide a reducedphase B power line current signal, RILS_(B), which is representative ofthe current waveform on the phase B power line 22, to the source ofdigital energy consumption signals 26. The second external PT 17 isconfigured to provide a reduced phase B power line voltage signal,RVLS_(B), which is representative of the voltage waveform on the phase Bpower line 22, to the source of digital energy consumption signals 26.As with the first external CT 14 and the first external PT 15, thesecond external CT 16 and the second external PT 17 introduce phaseerror and ratio error into the reduced phase B power line signals duringthe conversion. The phase error and the ratio error introduced by thefirst external CT 14 and the second external CT 16 need not beidentical. Likewise, the phase error and the ratio error introduced bythe first external PT 15 and the second external CT 17 need not beidentical.

Additionally, the third external CT 18 and the third external PT 19 areoperable to provide reduced phase C power line signals, RILS_(C) andRVLS_(C), to the source of digital energy consumption signals 26. Thethird external CT 18 and the third external PT 19 introduce their ownphase error and ratio error into the reduced phase C power line signals.

The source of digital energy consumption signals 26 is a device thatreceives power line signals and generates digital energy consumptionsignals therefrom. Digital energy consumption signals may suitablyinclude digital voltage signals and digital current signals. Digitalvoltage signals comprise a series of voltage samples that arerepresentative of the voltage waveform for each reduced power linevoltage signal. Similarly, digital current signals comprise a series ofcurrent samples that are representative of the current waveform for eachreduced power line current signal.

For example, with reference to FIG. 2, there is shown an exemplarydiagram of a reduced power line voltage waveform 40 over time. Thedigital voltage signals are the series of samples indicated by the bars42. The length of the each bar 42 represents the magnitude of thevoltage waveform 40 at an instant in time. Digitally, the verticalheight of each bar is represented as a digital voltage sample value.

In any event, the conversion of either reduced power line signals ornon-reduced power line signals into such digital energy consumptionsignals is well-known and may be carried out in a variety of ways.Typically, in a multiphase system such as the one illustrated in FIG. 1,one or more digital energy consumption signals will be produced for eachphase of the system. Thus, for example, the source of digital energyconsumption signals 26 may produce a digital voltage signal for phase A,a digital current signal for phase A, a digital voltage signal voltagesignal for phase B, a digital current signal for phase B, and so forth.

To generate the digital signals from the reduced power line signals, thesource of digital energy consumption signals 26 may suitably include oneor more sensor circuits, not shown, and one or more analog-to-digitalconversion devices. The sensor circuits, as discussed further above inthe background section, may be one or more devices that convert theincoming power line signals to voltage and current measurement signals.The A/D conversion devices convert the voltage and current measurementsignals into digital voltage signals and digital current signals,respectively. Suitable sensor circuits and A/D conversion devices arewell known. Exemplary sensor circuits and A/D conversion devices areshown in U.S. Pat. No. 5,544,089 and U.S. Pat. No. 6,112,158. In theembodiment described herein, a digital voltage signal and a digitalcurrent signal is produced for each phase of the three-phase power lines20, 22 and 24.

The source of digital energy consumption signals 26 is operablyconnected to provide the digital energy consumption signals to theprocessing circuit 28. The processing circuit 28 is generally operableto generate energy consumption data (e.g. values representative ofenergy consumed, a current power consumption level, and/or averagevoltage or current levels). The processing circuit 28 is furtheroperable to provide at least some of the energy consumption data to thedisplay 32, where the data may be reviewed for billing and/or trackingpurposes by utility personnel.

In accordance with the present invention, the processing circuit 28further cooperates with the memory 30 to operate as the compensationapparatus 12 that performs an adjustment based on error ratings of thetransformers 14, 15, 16, 17, 18 and 19. To this end, the memory 30stores data representative of a ratio error rating and a phase errorrating for each of the external transformers 14, 15, 16, 17, 18 and 19.The processing circuit 28 is configured to obtain at least one energyconsumption measurement value and adjust the at least one electricityconsumption measurement value using at least a portion of the storederror rating data.

For example, the processing circuit 28 may obtain an energy consumptionmeasurement value in the form of the generated energy consumption data,such as a watt-hour value. As discussed above, the processing circuit 28itself generates such energy consumption data from the received digitalenergy consumption signals. Once the processing circuit 28 generates theenergy consumption data, the processing circuit 28 the adjusts theenergy consumption data using the stored error rating data.

By using the stored data representative of the phase error ratings andthe ratio error ratings of the external transformers 14, 15, 16, 17, 18and 19 to adjust the energy consumption data, the energy consumptiondata will show improved accuracy over energy consumption data producedby meters that do not perform the adjustment discussed herein. Moreover,phase error ratings and ratio error ratings for external instrumenttransformers are readily available and indeed are typically affixed tothe transformer housing via a metal plate or the like. Thus,introduction of the error rating data into the memory 30 may be readilyaccomplished, for example, through the input device 34.

Although adjustment of the calculated energy consumption data asdiscussed above improves the accuracy of the energy consumption datagenerated by the meter 10, it is preferable that the processing circuit28 adjust the received digital energy consumption signals prior togenerating energy consumption data. For example, in such a preferredembodiment, the processing circuit 28 adjusts digital voltage signaland/or digital current signal using the stored error rating data. Inthis manner, the adjusted digital voltage and current signals may beused for a variety of subsequent calculations without requiringadjustments for each such calculation.

Regardless of how the adjustment is made, the memory 30 may be anymemory device such as random access memory, read only memory, orelectrically erasable read only memory (EEPROM). It is preferable toemploy an EEPROM as the memory 30 because it is both reprogrammable andnon-volatile. The memory 30 may also be employed to store other relevantdata within the meter. Stated another way, the memory 30 used inconnection with the present invention may readily be a part of memoryused for other purposes within the meter 10.

To achieve the above stated functions, the processing circuit 28 maysuitably comprise a microprocessor, digital signal processor, collectionof discrete digital components, or any combination of one or more of theabove elements. As discussed above, the processing circuit 28 asdescribed above operates in part as a portion of the compensationapparatus 12 according to the present invention and also in part as themeasurement circuit that converts digital energy consumption signalsinto energy consumption data. It will be appreciated that in thealternative, separate processing circuits may be used for such purposes.

In the exemplary embodiment described herein, the processing circuit 28preferably includes a digital signal processor and a microcontroller orthe like. Suitable processing circuits that employ a digital signalprocessor and one or more microcontrollers or microprocessors are shownin U.S. Pat. No. 5,544,089 and U.S. Pat. No. 6,112,158, for example.Preferably, the functions associated with the compensation apparatus 12of the processing circuit 28 are carried out primarily by the digitalsignal processor.

In operation, the meter 10 is provided to the vicinity of the load 36for installation. At some point, an installation technician must obtainthe ratio error ratings and the phase error ratings for the externaltransformers 14, 15, 16, 17, 18 and 19. To this end, the ratio errorratings and the phase error ratings are typically provided via indiciathat is affixed to the transformer housing, not shown. For example, theratio error rating and phase error rating of an instrument transformerare typically printed on a metal or plastic plate attached to thetransformer housing. However, in some embodiments, data sheets or moresubstantial error data for the transformers may be provided.

The installation technician must then store data representative of theratio error ratings and the phase error ratings in the memory 30 withinthe meter 10. To this end, the installation technician may provide theinformation through the input device 34. To do so, the meter 10 may needto be installed within the meter socket, not shown, so that it isconnected to receive operating power from one or more of the power lines20, 22, and 24. In such a case, the input device 34 may suitably be acommunication port that enables communication via optical or electricalsignals to a portable computer or meter programming device.

However, it will be noted that the data may be stored in the memory inother ways. For example, if the memory 30 is an EEPROM, the memory 30may be removed and then connected to an external device, not shown, thatwrites the error rating data to the memory 30. The memory 30 may then bereplaced into the meter 10. Such an operation may be performed withoutfirst installing the meter 10.

In any event, the installation technician must at some point install themeter 10 such that it is connected to the transformers 14, 15, 16, 17,18 and 19 and to the neutral line 38. Meter installation procedures arewell known to those of ordinary skill in the art.

After installation, the installation technician may cause the meter 10to begin metering operations. During normal metering operations, thetransformers 14, 16, and 18 receive the line signals LS_(A), LS_(B), andLS_(C) respectively. The line signals LS_(A), LS_(B), and LS_(C)constitute that actual electrical power signals that are provided to theload 36. The tranformers 14, 16, and 18 convert the lines signalsLS_(A), LS_(B), and LS_(C) to reduced power line current signalsRILS_(A), RILS_(B), and RILS_(C), and provide the reduced line currentsignals to the source of digital energy consumption signals 26. Thetranformers 15, 17, and 19 convert the lines signals LS_(A), LS_(B), andLS_(C) to reduced power line voltage signals RVLS_(A), RVLS_(B), andRVLS_(C), and provide the reduced line voltage signals to the source ofdigital energy consumption signals 26.

The source of digital energy consumption signals receives the RILS_(A),RILS_(B), RILS_(C), RVLS_(A), RVLS_(B), and RVLS_(C) signals andgenerates digital energy consumption signals therefrom. In the exemplaryembodiment described herein, the digital energy consumption signalsinclude VS_(A), VS_(B), VS_(C), IS_(A), IS_(B), and IS_(C), where VS_(x)is a digital voltage signal representative of the voltage waveform onthe phase x power line, and IS_(x) is a digital current signalrepresentative of the current waveform on the phase x power line.

The processing circuit 28 receives the digital energy consumptionsignals and converts the digital energy consumption signals into energyconsumption data. By way of example, the processing circuit 28 generatesperforms a real energy calculation by multiplying contemporaneousvoltage and current samples of each phase together and then summing theresulting products over time. For example, in the exemplary four wirewye system shown in FIG. 1, the processing circuit 28 may perform thefollowing calculations:WH _(A) =ΣVS _(A)(n)*IS _(A)(n) for n=1 to NWH _(B) =ΣVS _(B)(n)*IS _(B)(n) for n=1 to NWH _(C) =ΣVS _(C)(n)*IS _(C)(n) for n=1 to NTotal Energy=WH _(A) +WH _(B) +WH _(C)Where VS_(x)(n) is the nth voltage sample from phase x and IS_(x)(n) isthe nth voltage sample from phase x. The nth voltage sample and the nthcurrent sample are sampled substantially contemporaneously.

In accordance with the present invention, however, the processingcircuit 28 further performs an adjustment based on at least some of thestored data that is representative of the ratio error ratings or thephase error ratings of each of the transformers 14, 15, 16, 17, 18 and19. To this end, the processing circuit 28 obtains the stored errorrating data from the memory 30. As discussed above, the actualadjustment may be made to either the values WH_(x) or Total Energy.Thus, the processing circuit 28 could perform the following adjustment:Adj_WH_(A)=ƒ{WH_(A)(n), ra_err_IA, ph_err_IA, ra_err_VA, ph_err_VA}Adj_WH_(B)=ƒ{WH_(B)(n), ra_err_IB, ph_err_IB, ra_err_VB, ph_err_VB}Adj_WH_(C)=ƒ{WH_(C)(n), ra_err_IC, ph_err_IC, ra_err_VC, ph_err_VC }Total Energy=Adj _(—) WH _(A) +Adj _(—) WH _(B) +Adj _(—) WH _(C)Where ra_err_Ix is data representative of the ratio error rating of theCT connected to the phase x power line, ph_err_Ix is data representativeof the phase error rating of the CT connected to the phase x power line,ra_err_Vx is data representative of the ratio error rating of the PTconnected to the phase x power line, ph_err_Vx is data representative ofthe phase error rating of the PT connected to the phase x power line.Those of ordinary skill in the art may readily define a function ƒ thatincreases the accuracy of the calculated real energy based on either theratio error or the phase error.

By way of example, the value WH_(x) may simply be multiplied by acompensation factor that is based on the ratio error of the currenttransformer connected to phase x. In such a case, the values ra_err_Vx,ph_err_Ix and ph_err_Vx are disregarded. However, those of ordinaryskill in the art may readily incorporate any or all of the other errorratings into the energy consumption calculation if greater accuracy isdesired. It will be noted that in accordance with the present invention,the use of only some of the error ratings in the compensation functionwill provide at least some improved accuracy.

In any event, in contrast to the above described compensation function,in the preferred embodiment described herein, the processing circuit 28performs the adjustment on the voltage and/or current samples priorperforming the real energy calculation or any other energy consumptioncalculations. Accordingly, instead of performing the above describedfunctions, the processing circuit 28 receives the digital energyconsumption signals VS_(A), VS_(B), VS_(C), IS_(A), IS_(B), and IS_(C),and performs the adjustment on those signals.

In particular, the processing circuit first performs the followingoperations to adjust the current sample values of the digital currentsignal:Adj_IS_(A)(n)=ƒ(IS_(A)(n), ra_err_IA, ph_err_IA)Adj_IS_(B)(n)=ƒ(IS_(B)(n), ra_err_IB, ph_err_IB)Adj_IS_(C)(n)=ƒ(IS_(C)(n), ra_err_IC, ph_err_IC)Similar functions may be performed on the voltage samples to produceAdj_VS_(A)(n), Adj_VS_(B)(n), and Adj_VS_(C)(n). Exemplary functions forgenerating the adjusted samples are discussed below in connection withFIGS. 3 and 4.

With reference to the preferred embodiment described herein, theprocessing circuit 28 generates the energy consumption data using theadjusted digital energy consumption signals. For example, the processingcircuit 28 would perform the real energy calculation discussed aboveusing the following equations:WH _(A) =ΣAdj _(—) VS _(A)(n)*Adj _(—) IS _(A)(n) for n=1 to NWH _(B) =ΣAdj _(—) VS _(B)(n)*Adj _(—) IS _(B)(n) for n=1 to NWH _(C) =ΣAdj _(—) VS _(C)(n)*Adj _(—) IS _(C)(n) for n=1 to NTotal Energy=WH _(A) +WH _(B) +WH _(C)

One advantage of adjusting the digital energy consumption signals priorto generating energy consumption data is that no further adjustment isrequired to perform other energy consumption data calculations. Thus,for example, the per phase RMS voltage, per phase RMS current, reactiveenergy, apparent energy, power factor or other energy consumption datamay readily be calculated using the adjusted voltage and currentsamples.

FIGS. 3 and 4 shows in further detail exemplary block diagrams of theprocessing circuit operations for use in the compensation apparatus 12of FIG. 1 in accordance with the present invention. Those of ordinaryskill in the art may readily incorporate the operations shown in theblock diagrams of FIGS. 3 and 4 into a digital signal processor of theprocessing circuit 28 of FIG. 1. However, it will be noted that theoperations of FIGS. 3 and 4 may be carried out by processing circuits inmeters that are not also used to generate energy consumption data. Insuch cases, the outputs of FIGS. 3 and 4 would be provided to theprocessing circuit that generates the energy consumption data.

Referring again to the exemplary embodiment described herein, the blockdiagram 100 of FIG. 3 shows the operations performed by the processingcircuit 28 according to the present invention to adjust the currentsample values IS_(x)(n) of phase x of a digital energy consumptionsignal. While the example is shown in relation to a single phase x, itwill be noted that the block diagram 100 may readily be expanded toperform the operations on the current samples of three phases.

In general, the processing circuit 28 performs the operations of theblock diagram 100 to convert the raw current sample values IS_(x)(n)into adjusted current sample values Adj_IS_(x)(n) using datarepresentative of the ratio error rating of the external transformer onthe phase x power line. The processing circuit 28 further adjusts thecurrent sample values IS_(x)(n) using internal calibration information.The internal calibration information comprises a calibration factor thatcompensates for various error sources within the meter, for example,phase or magnitude errors caused by the sensor devices within the sourceof digital electricity consumption signals 26. Because the input currentsample values IS_(x)(n) are adjusted for both externally and internallygenerated errors, the value Adj_IS_(x)(n) more accurately reflects thecurrent waveform on the relevant power line for the purposes of energyconsumption calculations.

To this end, the current sample values IS_(x)(n) are provided to acorrection multiplier 102. The correction multiplier 102 also receives aphase x current correction factor corr_I_x from a correctionpre-multiplier 104. The correction multiplier 102 multiplies eachvoltage sample value IS_(x)(n) by corr_I_x to generate the output valueAdj_IS_(x)(n). The value corr_I_x represents the combined calibrationfactors generated from the internal calibration information Cal_Adj andthe external transformer ratio error compensation of the presentinvention, CT_R_Adj_x.

The internal calibration information may be generated as is known in theart. For example, the internal calibration information may be generatedusing data from a calibration operation in which a known amount ofenergy is provided to the meter 10 in the factory or otherwise. In thecalibration operation, the amount of energy consumption actuallymeasured by the meter 10 is compared to the known amount of energyprovided to the meter 10 to determine the internal calibration factor.Such operations are well known in the art. It will be appreciated,however, that if high quality, low tolerance sensor circuitry isemployed, there may be no need for correction using internal calibrationinformation. In such a case, the compensation pre-multiplier 104 wouldnot be necessary and the compensation multiplier 102 would be connecteddirectly to receive the external CT ratio error compensation valueCT_R_Adj_x.

In either event, the ratio error compensation value CT_R_Adj_x isderived from the ratio error ratings for current of the transformer onphase x. In particular, as discussed above, transformer errors are oftenspecified in terms of, among other things, ratio error ratings. Forexample, a current ratio error rating may identify the differencebetween the current magnitude actually produced by the second winding ofthe transformer and the expected current magnitude. The current ratioerror rating is typically given as a factor in relation to the averagecurrent level. Thus, multiplication of the measured RMS current by theratio error rating would produce the actual current level. Becauseexternal transformers are relatively accurate, it is not uncommon for aratio error rating to be in the range of 0.995 to 1.005.

Thus, if the ratio error rating for the relevant transformer is given asa single or average number, for example, 1.0003, then the errorcompensation value CT_R_Adj_x may suitably be 1.0003. As a result, theprocessing circuit 28 would generate the output current sample values,Adj_IS_(x)(n) using the following equation:Adj _(—) IS _(X)(n)=CT _(—) R _(—) Adj _(—) x*Cal _(—) Adj*IS _(x)(n)where Cal_Adj is the internal calibration information andCT_R_Adj_x=K1,where K1 is a constant based on the ratio error rating, which in theabove example would simply be a normalized value of 1.0003.Normalization is typically required to translate the specific ratioerror rating into the value range used by the specific processingsystem. By way of example, the processing circuit 28 may employ integerarithmetic and thus require translation into numbers suitable for use ininteger arithmetic.

It will be appreciated, however, that the ratio error of a currenttransformer is seldom a constant value. Instead, the ratio errortypically varies as a function of load current. Accordingly, ratio errorratings for external current transformers are often published to includeat least one “low load level” rating and at least one “high load level”rating. “Low load” is known in the industry to be current levels thatare approximately 10% of the “high load”. High load, by contrast, is aknown function of the overall current capacity of the transformer. Suchrelationships are known in the art.

Because the ratio error is typically not constant, it is desirable touse a dynamic compensation factor that changes as a function of the loadlevel or average current level. Using the high load ratio error ratingand the low load ratio error rating, the compensation factor CT_R_Adj_xmay be formulated as a function of RMS current (or average current).Such an expression would be:CT_R_Adj_x=ƒ[HLL_x_R, LLL_x_R, Ix_(RMS)],where HLL_x_R is the high load ratio error rating and LLL_x_R is the lowload ratio error rating for the relevant CT on the power line x. Thefunction ƒ could, for example, be a linear function itself with a slopedefined by the difference between the high load level ratio error ratingand the low load level ratio error rating. In other words, thecompensation factor could be expressed asCT _(—) R _(—) Adj _(—) x=KI+KS*Ix _(RMS)where KS is derived fromKS=A*[(HLL _(—) x _(—) R−LLL _(—) x _(—) R)/(HLL _(—) x _(—) I _(RMS)−LLL _(—) x _(—) I _(RMS))]where, HLL_x_I_(RMS) is the high load level RMS current, LLL_x_I_(RMS)is the low load level RMS current, and A is a system-specificnormalization factor. KI is a y-intercept value which may readily bederived by those of ordinary skill in the art by plotting the high loadlevel and low load level error ratings against the high and low loadlevel RMS currents.

It is noted that, as discussed above, the values HLL_x_R, LLL_x_R,HLL_x_I_(RMS), and LLL_x_I_(RMS) are all typically available directlythrough labeling on the transformer itself. Accordingly, the actualvalues LLL_x_I_(RMS), HLL_x_I_(RMS), LLL_x_R, and HLL_x_R may be storedin the memory 30 by the technician during meter installation asdiscussed above.

In addition, it has further been observed that the variance of theactual ratio error, determined empirically, does not vary in a strictlylinear relationship with respect to average load current. FIG. 5 showsexemplary test results showing the ratio error as a function of RMS loadcurrent level. The test results of FIG. 5 shows a first curve 302 of theactual measured ratio error percentage as a function of input current onan exemplary transformer wherein the input power signal had a powerfactor of 1.0. Similar response curves 304 and 306 were observed atother power factors.

Using approximate curve fitting, it is observable that the error ratiorating of the transformers varies in an inverse relationship to current,or in other words, as a factor of (RMS current)^(−P), where P is apositive number, for example, 1.

Accordingly, in accordance with a preferred embodiment of the presentinvention, the compensation factor CT_R_Adj_x is a function that variesin an inverse relationship to the average or RMS current level. In theembodiment shown in FIG. 3, the inverse relationship is generatedbasically as the following equation.CT _(—) R _(—) Adj _(—) x=KA _(x)+(KB _(x) /Ix _(RMS))Where KA_(x) and KB_(x) are constants derived from the high and low loadlevel error ratings for the transformer coupled to phase x.

Referring again to FIG. 3, the above general equation is carried out inthe following manner. An RMS sensor 106 generates the Ix_(RMS)information. To this end, the RMS sensor 106 receives the adjustedcurrent sample values IS_(X)(n) and performs a typical RMS calculationto generate Ix_(RMS). A current ratio calculation block 108 receives thecalculated Ix_(RMS) and uses the value to generate an interim currentratio value Amps_Ratio_x that is generated from the equationAmps_Ratio_(—) x=LLL _(—) x _(—) I _(RMS) /Ix _(RMS)If, however, Ix_(RMS) is zero, then Ix_(RMS) is replaced by a de minimusvalue to avoid a divide by zero error. To carry out the foregoingAmps_Ratio_x determination, the current ratio calculation block 108 alsoreceives, in addition to Ix_(RMS), the low load RMS current level forthe transformer, LLL_x_I_(RMS), and the given de minimus current level.It can be seen from the above equation that the interim valueAmps_Ratio_x carries the inverted RMS current or (RMS Current)⁻¹information.

The current ratio calculation block 108 provides the calculated interimvalue Amps_Ratio_x to the ratio adjustment calculation block 110. Theratio adjustment calculation block 110 uses the Amps_Ratio_x value andtwo other interim values, one including primarily information derivedfrom the high load ratio error rating for the transformer connected tophase x and the other including primarily information derived from thelow load ratio error rating of the transformer connected to phase x. Inparticular, the interim values are Int_H_x and Int_L_x. Those values aregenerate as follows:${{Int\_ H}{\_ x}} = \frac{\left\lbrack {{{HLL\_ x}{\_ R}} - {\left( {{LLL\_ x}{{\_ I}_{RMS}/{HLL\_ x}}{\_ I}_{RMS}} \right)*{LLL\_ x}{\_ R}}} \right\rbrack}{\left( {1 - \left( {{LLL\_ x}{{\_ I}_{RMS}/{HLL\_ x}}{\_ I}_{RMS}} \right)} \right.}$ Int_L_x=LLL_x_RAs discussed above, HLL_x_R is the high load ratio error rating for thetransformer on phase x, LLL_x_R is the low load ratio error rating forthe transformer on phase x, LLL_x_I_(RMS) is the low load RMS currentfor the transformer, and HLL_x_I_(RMS) is the high load RMS current forthe transformer.

It will be noted that the actual values LLL_x_I_(RMS), HLL_x_I_(RMS),LLL_x_R, and HLL_x_R may be retrieved from the memory 30. However, inthe alternative, the processing circuit 28 may convert those valuesdirectly into Int_H_x and Int_L_x before storing the data into thememory 30.

In any event, the ratio adjustment calculation block 110 performs thefollowing calculation to produce the compensation factor CT_R_Adj_x:CT _(—) R _(—) Adj _(—) x=Int _(—) H _(—) x+[Amps_Ratio_(—) x*(Int _(—)Lx−Int _(—) H _(—) x)]Which from the above expands to show the inverse relationship toIx_(RMS):${{CT\_ R}{\_ Adj}{\_ x}} = {{{Int\_ H}{\_ x}} + \frac{\left\lbrack {{LLL\_ x}{\_ I}_{RMS}*\left( {{{Int\_ L}{\_ x}} - {{Int\_ H}{\_ x}}} \right)} \right\rbrack}{{Ix}_{RMS}}}$

The ratio adjustment calculation block 110 provides the compensationfactor CT_R_Adj_x to the compensation pre-multiplier 104 through aswitch 112. The switch 112 may be used to disable the compensation ifdesired. It is preferable that the meter user have the option to disablethe compensation when necessary. For example, disabling the compensationmay be necessary to perform an in service certification of the meter'saccuracy.

The block diagram 100 furthermore shows a phase adjustment calculationblock 114 that generates a phase compensation factor based in part onthe phase error ratings for the transformer x. As will be discussedfurther below, the phase compensation factor CT_Ph_Adj_x is applied toadjust the voltage sample values VS_(x)(n) in the exemplary embodimentherein. It will be noted that a phase adjustment may be applied toeither the current sample values or the voltage sample values, becauseit is the phase difference between the current waveform and the voltagewaveform that requires accuracy. In the exemplary embodiment herein, thephase compensation factor is applied to the voltage sample valuesVS_(x)(n) as discussed further below in connection with FIG. 4. However,the exemplary embodiment described herein may readily be modified toapply the phase compensation factor to the current sample valuesIS_(x)(n).

Application of the phase adjustment to the voltage samples is preferablebecause the adjustment is less subject to rounding errors. Inparticular, because current signals vary greatly and may becomerelatively small, a relatively small phase adjustment may be prone torounding errors. By contrast voltage signals are constant and relativelylarge.

In any event, the calculation of the phase compensation factor,CT_Ph_Adj_x, like the ratio compensation factor, CT_R_Adj_x, may beexpressed as a constant based on the phase error rating for thetransformer on the phase x power line. As with the ratio error ratingsdiscussed above, however, it is typical to provide two phase errorratings, one at high load currents and one at low load currents.Accordingly, similar to CT_R_Adj_x, the value CT_Ph_Adj_x is preferablydynamic and is a function of the load current, or Ix_(RMS). Thus, forexample, the phase compensation factor may be given by the followingequation:CT _(—) R _(—) Adj _(—) x=KI _(—) Ph _(—) x+KS _(—) Ph _(—) x*Ix _(Rms)where KS_Ph_x is derived fromKS _(—) Ph _(—) x=B*[(HLL _(—) x _(—) Ph−LLL _(—) x _(—) Ph)/(HLL _(—) x_(—) I _(RMS) −LLL _(—) I _(RMS))]where, HLL_x_Ph is the high load phase error rating, LLL_x_Ph is the lowload level phase error rating, and B is a system-specific normalizationfactor. KI_Ph_x is a y-intercept value which may readily be derived bythose of ordinary skill in the art.

However, similar to the ratio error ratings on external instrumenttransformers, it has been noted that the relationship between the actualphase error factor with respect to current level is not strictly linear.It has been observed that the phase error of transformers will vary inan inverse relationship to average or RMS current, or in other words, asa factor of (RMS current)^(−P), where P is a positive number, forexample, 1.

Accordingly, in accordance with a preferred embodiment of the presentinvention, the compensation factor CT_Ph_Adj_x is a function that variesin an inverse relationship to the average or RMS current level. In theembodiment shown in FIG. 3, the inverse relationship is generatedbasically as the following equation.CT _(—) Ph _(—) Adj _(—) x=KC _(x)+(KD _(x) /Ix _(RMS))where KC_(x) and KD_(x) are constants derived from the high and low loadlevel phase error ratings for the transformer coupled to phase x.

The above-described generalized equation is carried out by the phaseadjustment calculation block 114 in the following manner. The phaseadjustment calculation block 114 uses the Amps_Ratio_x value and twoother interim values, one including primarily information derived fromthe high load phase error rating for the transformer connected to phasex, HLL_x_Ph, and the other including primarily information derived fromthe low load ratio error rating of the transformer connected to phase x,LLL_x_Ph. In particular, the interim values are Int_Ph_H_x andInt_PH_L_x. Those values are generated as follows:${{Int\_ Ph}{\_ H}{\_ x}} = \frac{\left\lbrack {{{HLL\_ x}{\_ Ph}} - {\left( {{LLL\_ x}{{\_ I}_{RMS}/{HLL\_ x}}{\_ I}_{RMS}} \right)*{LLL\_ x}{\_ Ph}}} \right\rbrack}{\left( {1 - \left( {{LLL\_ x}{{\_ I}_{RMS}/{HLL\_ x}}{\_ I}_{RMS}} \right)} \right.}$ Int_Ph_L_x=LLL_x_PhAs discussed above, HLL_x_Ph is the high load ratio error rating for thetransformer on phase x, LLL_x_Ph is the low load ratio error rating forthe transformer on phase x, LLL_x_Ix_(RMS) is the low load RMS currentfor the transformer, and HLL_x_I_(RMS) is the high load RMS current forthe transformer. All those values are typically available for a giveninstrument transformer.

As with the ratio error information, the values LLL_x_I_(RMS),HLL_x_I_(RMS), LLL_x_Ph, and HLL_x_Ph, which may be normalized for theprocessing unit 28, may be retrieved from the memory 30. However, in thealternative, the processing circuit 28 may convert those values directlyinto Int_Ph_H_x and Int_Ph_L_x before storing the data into the memory30.

In any event, the phase adjustment calculation block 114 performs thefollowing calculation to produce the compensation factor CT_Ph_Adj_x:CT _(—) Ph _(—) Adj _(—) x=Int _(—) Ph _(—) H _(—) x+[Amps_Ratio_(—)x*(Int _(—) Ph _(—) L _(—) x−Int _(—) Ph _(—) H _(—) x)]Which from the above expands to show the inverse relationship toIx_(RMS):${{CT\_ Ph}{\_ Adj}{\_ x}} = {{{Int\_ Ph}{\_ H}{\_ x}} + \frac{\left\lbrack {{LLL\_ x}{\_ I}_{RMS}*\left( {{{Int\_ Ph}{\_ L}{\_ x}} - {{Int\_ Ph}{\_ H}{\_ x}}} \right)} \right\rbrack}{{Ix}_{RMS}}}$

The phase adjustment calculation block 114 provides the compensationfactor CT_Ph_Adj_x to the phase adjust summation device 208 in FIG. 4through a switch 212.

Referring to FIG. 4, the block diagram 200 of FIG. 4 shows the generaloperations of the processing circuit 28 that are executed to adjustvoltage sample values VS_(x)(n) received from the source of digitalelectrical consumption signals 26. In general, the processing circuit 28performs the operations of the block diagram 200 to convert the rawvoltage sample values VS_(x)(n) into adjusted voltage sample valuesAdj_VS_(x)(n) using data representative of the various error ratings ofthe external potential transformers on the phase x power line. Moreover,as discussed above, the processing circuit 28 also uses the determinedcurrent transformer phase compensation factor CT_Ph_Adj_x to generatethe adjusted voltage sample values to compensate for the externalcurrent transformer phase error.

The processing circuit 28 further adjusts the voltage sample valuesVS_(x)(n) using internal calibration information. As discussed above,internal calibration information comprises a calibration factor thatcompensates for various error sources within the meter, for example,phase or magnitude errors in the sensor devices within the source ofdigital electricity consumption signals 26. Thus, the valueAdj_VS_(x)(n) more accurately reflects the voltage waveform on therelevant power line for the purposes of energy consumption calculations.

In particular, the voltage sample values VS_(x)(n) are provided to acorrection multiplier 202. The correction multiplier 202 also receives aphase x voltage correction factor corr_V_x from a correctionpre-multiplier 204. The correction multiplier 202 multiplies eachvoltage sample value VS_(x)(n) by corr_V_x to generate an interim value,Int_VS_(x)(n), which is in turn provided to a phase shifter 205 togenerate the output value Adj_VS_(x)(n). The value corr_V_x representsthe combined calibration factors of the internal calibrationinformation, Cal_V_Adj, and the external potential transformer ratioerror compensation.

The internal calibration information may be generated as is known in theart. As above, it will be appreciated that the use of high quality, lowtolerance sensor circuitry may eliminate the need for correction usinginternal calibration information. In such a case, the compensationpre-multiplier 204 would not be necessary and the compensationmultiplier 202 would be connected to receive directly the externalpotential transformer ratio error compensation value PT_R_Adj_x.

In either event, the ratio error compensation value PT_R_Adj_x isderived from the ratio error ratings for the potential transformer onphase x. In particular, as discussed above, transformer errors are oftenspecified in terms of, among other things, ratio error ratings. Forexample, a potential ratio error rating may identify the differencebetween the voltage magnitude actually produced by the second winding ofthe transformer and the expected voltage magnitude. The voltage ratioerror rating is typically given as a factor in relation to the averagevoltage level. Thus, multiplication of the measured RMS voltage by theratio error rating would produce the actual voltage magnitude. Becauseexternal transformers are relatively accurate, it is not uncommon for aratio error rating to be in the range of 0.995 to 1.005.

Thus, if the ratio error rating for the relevant transformer is given asa single or average number, for example, 1.0002, then the errorcompensation value PT_R_Adj_x may suitably be simple 1.0002. As aresult, the processing circuit 28 would generate the output currentsample values, Int_VS_(x)(n) using the following equation:Int _(—) VS _(x)(n)=PT _(—) R _(—) Adj _(—) x*Cal _(—) Adj _(—) V*VS_(x)(n)where Cal_Adj_V is the internal calibration information andPT_R_Adj_x=K2,where K2 is a constant based on the ratio error rating, which in theabove example would simply be a normalized value of 1.0002.

Unlike the current transformer ratio correction factor CT_R_Adj_xdiscussed above in connection with FIG. 3, the potential transformerratio correction factor PT_R_Adj_x provides sufficient correction as aconstant factor. In particular, as opposed to current in the powerlines, the voltage in the power lines should remain substantiallyconstant, as is known in the art. Accordingly, it is not necessary tovary the correction factor as a function of average line voltage.

It is noted that because the potential does not vary significantly in aninstrument potential transformer, such transformers are typicallyprovided with only one ratio error rating. That ratio error rating,after normalization, may be used directly as the potential transformerratio correction factor PT_R_Adj_x.

The potential transformer ratio correction factor PT_R_Adj_x may beprovided to the compensation pre-multiplier 204 through a switch 206 toallow the operator to disable the ratio compensation if desired.

The processing circuit 28 in the execution of the block diagram 200further generates a potential transformer phase correction factorPT_Ph_Adj_x. As with the ratio error rating, the potential transformerphase correction factor PT_Ph_Adj_x may be a constant factor deriveddirectly from the potential transformer phase error rating, PT_Ph_x. Aswith the current transformer phase error ratings, the potentialtransformer phase error rating, PT_Ph_x is typically provided in unitsof minutes, or sixtieths of a degree.

In any event, the potential transformer phase correction factor,PT_Ph_Adj_x is provided to the positive input of a summation device 208through disabling switch 210. Likewise, the current transformer phasecorrection factor, CT_Ph_Adj_x, generated in the execution of the flowdiagram 100 of FIG. 3, is provided to the negative input of thesummation device 208 through a disabling switch 212.

The summation device 208 generates a composite phase correction valuePh_corr_x for the phase x voltage sample values, VS_(x)(n). Thecomposite phase correction value Ph_corr_x is then provided to a phaseshifter 205 which then adjusts the phase using Ph_corr_x.

To this end, the phase shifter 205 may suitably take the form of theexemplary block diagram 250 on FIG. 6. In particular, the phase shifter205 in FIG. 6 includes a voltage sample input 252, a low pass digitalfilter 254, a multiplier 256, a phase correction input 258, and asummation device 260.

The voltage sample input 252 receives the interim voltage sample valuesInt VS_(x)(n) from the compensation multiplier 202 of FIG. 4 andprovides the interim sample values to the low pass digital filter 254and the summation device 260. The low pass digital filter 254 providesthe phase-shifted and filtered voltage sample values to a first input ofthe multiplier 256. To this end, the low pass digital filter 254 maysuitably be a single pole, IIR low pass filter having approximately a 1Hz corner frequency (in a 60 Hz power system). As a result, the delayedsamples are almost 90° out of phase.

The phase correction input 258 is coupled to provide the composite phasecorrection value Ph_corr_x to the second input of the multiplier 256.The multiplier 256 multiplies Ph_corr_x by the phase-shifted voltagesample values to generate a phase shift value, Ph_shift. The multiplier256 provides the phase shift value Ph_shift to the summation device 260.The summation device 260 sums the interim voltage sample values providedby the voltage sample input 252 with Ph_shift to effectuate the phaseshift of the interim voltage sample values. The phase shifted interimsample values constitute the final adjusted voltage sample valuesAdj_VS_(x)(n).

Thus, the present invention effectuates a compensation for bothpotential transformer error and current transformer error that arecoupled external to the meter 10. As is known in the art, such externaltransformers typically provide error rating information. The presentinvention allows the use of that error rating to generate a compensationfactors to increase the reliability of the generated meter data. It willbe noted that at least some of the benefits of the present inventionwill be realized even if only one or two of the above describedcompensation factors is employed. For example, employing only thecurrent transformer ratio compensation factor in accordance with thepresent invention will provide at least some reduction in meteringerror. Likewise, at least some effort of the invention is realized ifthe compensation is only based on the current transformer phase errorrating, or either of the potential transformer error ratings. Moreover,it may be useful in retrofit or modification applications to employ thecompensation method and apparatus described herein for certain errorswhile employing existing compensation arrangements to compensate forother errors.

In any event, it will be appreciated that the embodiments describedherein are merely exemplary, and that those of ordinary skill in the aremay readily devise their own implementations that incorporate theprinciples of the present invention and fall within the spirit and scopethereof.

1. An arrangement for use in an electricity meter, the electricity meteroperably coupled through an external transformer to measure electricityconsumption on a power line, the arrangement operable to compensate formeasurement errors, the arrangement comprising: a) a source of digitalmeasurement signals comprising an internal sensor circuit and ananalog-to-digital conversion circuit, the internal sensor circuitconfigured to convert power line signals received from the externaltransformer to measurement signals, the analog-to-digital conversioncircuit configured to receive the measurement signals from the sensorcircuit and convert the measurement signals to digital measurementsignals; b) a memory storing data representative of at least one errorrating for the external transformer; c) a processing circuit operablycoupled to the source of digital measurement signals to receive digitalmeasurement signals therefrom; the processing circuit operable to obtainat least one electricity consumption measurement value corresponding toat least a part of the digital measurement signals, and adjust the atleast one electricity consumption measurement value using at least aportion of the stored data.
 2. The arrangement of claim 1 wherein the atleast one electricity consumption measurement value comprises acalculated energy consumption value.
 3. The arrangement of claim 1wherein the at least one electricity consumption measurement valuecomprises at least one of a sampled current value or a sampled voltagevalue.